Best practices and security considerations are moved to solidity-security-guidelines.
This document is a cheatsheet for Solidity that you can use to write Smart Contracts for Ethereum based blockchain.
This guide is not intended to teach you Solidity from the ground up, but to help developers with basic knowledge who may struggle to get familiar with Smart Contracts and Blockchain because of the Solidity concepts used.
Note: If you have basic knowledge in JavaScript, it is more easy to learn Solidity.
- Solidity Cheatsheet and Best practices
pragma solidity ^0.4.18; will compile with a compiler version > 0.4.0 and < 0.5.0.
import "filename";
import * as symbolName from "filename"; or import "filename" as symbolName;
import {symbol1 as alias, symbol2} from "filename";
bool : true or false
Operators:
- Logical :
!(logical negation),&&(AND),||(OR) - Comparisons :
==(equality),!=(inequality)
Unsigned : uint8 | uint16 | uint32 | uint64 | uint128 | uint256(uint)
Signed : int8 | int16 | int32 | int64 | int128 | int256(uint)
Operators:
- Comparisons:
<=,<,==,!=,>=and> - Bit operators:
&,|,^(bitwise exclusive or) and~(bitwise negation) - Arithmetic operators:
+,-, unary-, unary+,*,/,%,**(exponentiation),<<(left shift) and>>(right shift)
address: Holds an Ethereum address (20 byte value).
Operators:
- Comparisons:
<=,<,==,!=,>=and>
Methods:
<address>.balance (uint256): balance of the Address in Wei
<address>.transfer(uint256 amount): send given amount of Wei to Address, throws on failure<address>.send(uint256 amount) returns (bool): send given amount of Wei to Address, returns false on failure
<address>.call(...) returns (bool): issue low-level CALL, returns false on failure
<address>.delegatecall(...) returns (bool): issue low-level DELEGATECALL, returns false on failure
Delegatecall uses code of the target address and all other aspects (storage, balance, ...) are taken from the calling contract. The purpose of delegatecall is to use library code which is stored in another contract. The user has to ensure that the layout of storage in both contracts is suitable for delegatecall to be used.
contract A {
uint value;
address public sender;
address a = "0x"; // address of contract B
function makeDelegateCall(uint _value) {
a.delegatecall(bytes4(keccak256("setValue(uint)")), _value); // Value of A is modified
}
}
contract B {
uint value;
address public sender;
setValue(uint _value) {
value = _value;
sender = msg.sender; // msg.sender is preserved in delegatecall. It was not available in callcode.
}
}
gas() option is available for call, callcode and delegatecall. value() option is not supported for delegatecall.
<address>.callcode(...) returns (bool): issue low-level CALLCODE, returns false on failure
Prior to homestead, only a limited variant called
callcodewas available that did not provide access to the originalmsg.senderandmsg.valuevalues.
Arrays can have fixed size or they can be dynamic.
uint[] dynamicSizeArray;
uint[7] fixedSizeArray;
bytes1(byte), bytes2, bytes3, ..., bytes32.
Operators:
Comparisons: <=, <, ==, !=, >=, > (evaluate to bool)
Bit operators: &, |, ^ (bitwise exclusive or), ~ (bitwise negation), << (left shift), >> (right shift)
Index access: If x is of type bytesI, then x[k] for 0 <= k < I returns the k th byte (read-only).
Members
.length: read-only
bytes: Dynamically-sized byte array. It is similar to byte[], but it is packed tightly in calldata. Not a value-type!
string: Dynamically-sized UTF-8-encoded string. It is equal to bytes but does not allow length or index access. Not a value-type!
Enum works just like every other language.
enum ActionChoices {
GoLeft,
GoRight,
GoStraight,
SitStill
}
ActionChoices choice = ActionChoices.GoStraight;
New types can be declared using struct.
struct Funder {
address addr;
uint amount;
}
Funder funders;
Declared as mapping(_KeyType => _ValueType)
Mappings can be seen as hash tables which are virtually initialized such that every possible key exists and is mapped to a value.
key can be almost any type except for a mapping, a dynamically sized array, a contract, an enum and a struct. value can actually be any type, including mappings.
Most of the control structures from JavaScript are available in Solidity except for switch and goto.
ifelsewhiledoforbreakcontinuereturn? :
function (<parameter types>) {internal|external|public|private} [pure|constant|view|payable] [returns (<return types>)]
public- Accessible from this contract, inherited contracts and externallyprivate- Accessible only from this contractinternal- Accessible only from this contract and contracts inheriting from itexternal- Cannot be accessed internally, only externally. Recommended to reduce gas. Access internally withthis.f.
Parameters are declared just like variables and are memory variables.
function f(uint _a, uint _b) {}
Output parameters are declared after returns keyword
function f(uint _a, uint _b) returns (uint _sum) {
_sum = _a + _b;
}
Output can also be specified using return statement. In that case, we can omit parameter name returns (uint).
Multiple return types are possible with return (v0, v1, ..., vn).
Function that has same name as contract. Executed during contract deployment.
contract C {
address owner;
uint status;
function C(uint _status) {
owner = msg.sender;
status = _status;
}
}
Functions of the current contract can be called directly(internally - via jumps) and also recursively
contract C {
function funA() returns (uint) {
return 5;
}
function FunB(uint _a) returns (uint ret) {
return funA() + _a;
}
}
this.g(8); and c.g(2); (where c is a contract instance) are also valid function calls, but, the function will be called “externally”, via a message call.
.gas()and.value()can also be used with external function calls.
Function call arguments can also be given by name in any order as below.
function f(uint a, uint b) { }
function g() {
f({b: 1, a: 2});
}
Parameters will be present on the stack, but they are not accessible.
function f(uint a, uint) returns (uint) {
return a;
}
Pass function as a parameter to another function. Similar to callbacks and delegates
pragma solidity ^0.4.18;
contract Oracle {
struct Request {
bytes data;
function(bytes memory) external callback;
}
Request[] requests;
event NewRequest(uint);
function query(bytes data, function(bytes memory) external callback) {
requests.push(Request(data, callback));
NewRequest(requests.length - 1);
}
function reply(uint requestID, bytes response) {
// Here goes the check that the reply comes from a trusted source
requests[requestID].callback(response);
}
}
contract OracleUser {
Oracle constant oracle = Oracle(0x1234567); // known contract
function buySomething() {
oracle.query("USD", this.oracleResponse);
}
function oracleResponse(bytes response) {
require(msg.sender == address(oracle));
}
}
Modifier can automatically check a condition prior to executing the function.
modifier onlyOwner {
require(msg.sender == owner);
_;
}
function close() onlyOwner {
selfdestruct(owner);
}
Functions can be declared view or constant in which case they promise not to modify the state, but can read from them.
function f(uint a) view returns (uint) {
return a * b; // where b is a storage variable
}
The compiler does not enforce yet that a
viewmethod is not modifying state.
Functions can be declared pure in which case they promise not to read from or modify the state.
function f(uint a) pure returns (uint) {
return a * 42;
}
Functions that receive Ether are marked as payable function.
A contract can have exactly one unnamed function. This function cannot have arguments and cannot return anything. It is executed on a call to the contract if none of the other functions match the given function identifier (or if no data was supplied at all).
function() {
// Do something
}
Contract can be created from another contract using new keyword. Source of the contract has to be known in advance.
contract A {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
}
}
contract C {
address a;
function f(uint _a) {
a = new A();
}
}
Solidity supports multiple inheritance and polymorphism.
contract owned {
function owned() { owner = msg.sender; }
address owner;
}
contract mortal is owned {
function kill() {
if (msg.sender == owner) selfdestruct(owner);
}
}
contract final is mortal {
function kill() {
super.kill(); // Calls kill() of mortal.
}
}
contract A {}
contract B {}
contract C is A, B {}
contract A {
uint a;
function A(uint _a) { a = _a; }
}
contract B is A(1) {
function B(uint _b) A(_b) {
}
}
Contracts that contains implemented and non-implemented functions. Such contracts cannot be compiled, but they can be used as base contracts.
pragma solidity ^0.4.0;
contract A {
function C() returns (bytes32);
}
contract B is A {
function C() returns (bytes32) { return "c"; }
}
Interfaces are similar to abstract contracts, but they have restrictions:
- Cannot have any functions implemented.
- Cannot inherit other contracts or interfaces.
- Cannot define constructor.
- Cannot define variables.
- Cannot define structs.
- Cannot define enums.
pragma solidity ^0.4.11;
interface Token {
function transfer(address recipient, uint amount);
}
Events allow the convenient usage of the EVM logging facilities, which in turn can be used to “call” JavaScript callbacks in the user interface of a dapp, which listen for these events.
Up to three parameters can receive the attribute indexed which will cause the respective arguments to be searched for
All non-indexed arguments will be stored in the data part of the log.
pragma solidity ^0.4.0;
contract ClientReceipt {
event Deposit(
address indexed _from,
bytes32 indexed _id,
uint _value
);
function deposit(bytes32 _id) payable {
Deposit(msg.sender, _id, msg.value);
}
}
Libraries are similar to contracts, but they are deployed only once at a specific address and their code is used with delegatecall (callcode).
library arithmatic {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
}
}
contract C {
uint sum;
function f() {
sum = arithmatic.add(2, 3);
}
}
using A for B; can be used to attach library functions to any type.
library arithmatic {
function add(uint _a, uint _b) returns (uint) {
return _a + _b;
}
}
contract C {
using arithmatic for uint;
uint sum;
function f(uint _a) {
sum = _a.add(3);
}
}
assert(bool condition): throws if the condition is not met - to be used for internal errors.require(bool condition): throws if the condition is not met - to be used for errors in inputs or external components.revert(): abort execution and revert state changes
function sendHalf(address addr) payable returns (uint balance) {
require(msg.value % 2 == 0); // Only allow even numbers
uint balanceBeforeTransfer = this.balance;
addr.transfer(msg.value / 2);
assert(this.balance == balanceBeforeTransfer - msg.value / 2);
return this.balance;
}
Catching exceptions is not yet possible.
block.blockhash(uint blockNumber) returns (bytes32): hash of the given block - only works for 256 most recent blocks excluding currentblock.coinbase (address): current block miner’s addressblock.difficulty (uint): current block difficultyblock.gaslimit (uint): current block gaslimitblock.number (uint): current block numberblock.timestamp (uint): current block timestamp as seconds since unix epochnow (uint): current block timestamp (alias forblock.timestamp)
msg.data (bytes): complete calldatamsg.gas (uint): remaining gasmsg.sender (address): sender of the message (current call)msg.sig (bytes4): first four bytes of the calldata (i.e. function identifier)msg.value (uint): number of wei sent with the messagetx.gasprice (uint): gas price of the transactiontx.origin (address): sender of the transaction (full call chain)
addmod(uint x, uint y, uint k) returns (uint): compute (x + y) % k where the addition is performed with arbitrary precision and does not wrap around at 2**256.mulmod(uint x, uint y, uint k) returns (uint): compute (x * y) % k where the multiplication is performed with arbitrary precision and does not wrap around at 2**256.keccak256(...) returns (bytes32): compute the Ethereum-SHA-3 (Keccak-256) hash of the (tightly packed) argumentssha256(...) returns (bytes32): compute the SHA-256 hash of the (tightly packed) argumentssha3(...) returns (bytes32): alias to keccak256ripemd160(...) returns (bytes20): compute RIPEMD-160 hash of the (tightly packed) argumentsecrecover(bytes32 hash, uint8 v, bytes32 r, bytes32 s) returns (address): recover the address associated with the public key from elliptic curve signature or return zero on error (example usage)
this (current contract’s type): the current contract, explicitly convertible to Addressselfdestruct(address recipient): destroy the current contract, sending its funds to the given Addresssuicide(address recipient): alias to selfdestruct. Soon to deprecate.